1. Field of the Invention
The present invention relates to processes for recovering paraxylene from mixtures of aromatic compounds and more particularly to such processes which isomerize and recycle the raffinate from an adsorption process for recovering paraxylene.
2. Description of the Prior Art
In the processing of hydrocarbons, such as oil, it is desirable to form, concentrate and remove paraxylene since paraxylene is a relatively more valuable hydrocarbon. A typical hydrocarbon mixture containing paraxylene and which is usually processed to obtain concentrated paraxylene is reformate from thermal or catalytic reforming processes. These reforming processes are used to form aromatic hydrocarbons from non-aromatic hydrocarbons.
In processing reformate to concentrate and separate paraxylene it is first subjected to solvent extraction to remove non-aromatics and then fractionated to form a C.sub.8 aromatic concentrated material. This material is then subjected to an adsorption process to remove an essentially pure paraxylene product from the C.sub.8 aromatic concentrated material. An adsorption process for paraxylene now used in the art is known as the Parex process. The raffinate from this adsorption process is isomerized to produce paraxylene from the C.sub.8 isomers in the raffinate and the product of isomerization is recycled to the adsorption process for removing the newly formed paraxylene. Usually the isomerized raffinate is fractionated to concentrate the C.sub.8 aromatics and then recycled to the Parex unit.
A problem with the paraxylene formation, concentration and removal processes of the prior art has been that isomerization converts some of the aromatic compounds to non-aromatics such as naphthenes and paraffins. These materials are difficult to fractionate from the recycle to the adsorption process and so build up in the recycle stream. This build up of non-aromatics increases until the fractionization can prevent any further build up. This increased concentration reduces the efficiency of the adsorption and isomerization and reduces the yield of paraxylene from the process as a whole.
It is accordingly an object of the present invention to provide a process which improves the recovery of paraxylene from hydrocarbon mixtures such as reformate. It is also an object of the present invention to provide such an improved process which does not require significant new equipment or expensive steps.